In general, the otter boards are used to open the mouth of a trawl net in a trawling operation. There are conventionally known otter boards of various types such as a single-plate and multi-plate types, but it is necessary to adjust the spreading force depending upon the fishery purpose. To adjust the spreading force of the otter boards, it is a conventional practice to change the angle of elevation of the otter boards with respect to a towing direction, or to change the area of an otter board body.
If the angle of elevation of the otter board is changed, however, the stability of opening the mouth of the net is degraded and hence, this method has conventionally been less used. In addition, to change the area of the otter boards, it is normally necessary to provide a vessel with otter boards of several types having different areas, resulting in an increased cost and a long time required for replacement of the otter boards, with a danger attendant on the replacing operation. Particularly, in multi-plate type otter boards disclosed in Japanese Patent Application Laid-open No. 99881/74 and Japanese Utility Model Application Laid-open No. 106458/85, individual plates of the otter boards are secured together, resulting in an increased occupying space. Further, the above disadvantage is more serious.
Thereupon, for overcoming these problems, proposals, for example, as disclosed in Japanese Patent Publication Nos. 16858/76 and 42711/77 have been made.
In these proposals, drainage channels or slits are made through a thicker otter board, so that changing of the area of the opened channels or slits by the operation of an on-off valve or an otter flap causes the amount of water entering the individual channels or slits to be adjusted, thereby adjusting the spreading force of the otter board.
In these methods, however, the amount of spreading force is less changed and hence, it is difficult to adjust the spreading force in a wider range, because the opened area of the individual channel or slit in the inner surface of the otter board is fixed, and the on-off valve or flap is provided for adjusting the amount of water flowing into deeper portion of each channel or slit.